1. Field of the Invention
The present invention relates to a navigation apparatus, which is mounted in a movable body, such as a vehicle or the like, and detects a present position, a travel direction, a travel distance and the like of the movable body while it is moving, to indicate the present position of the movable body on a map shown on a display, and accordingly assists the movement to a destination of the movable body.
2. Description of the Related Art
As a measuring apparatus for a movable body such as a vehicle, an airplane, a ship or the like, there is a so-called navigation apparatus which superimposes and shows, on a map including the present position of the movable body, a position mark indicating the present position of the movable body, to accordingly perform a route guidance to a destination. Among the navigation apparatuses, a vehicle navigation apparatus mounted in the vehicle is roughly classified into a self sustained navigation apparatus and a GPS (Global Positioning System) navigation apparatus.
The former type determines a travel distance of the vehicle by using a travel distance sensor equipped in the vehicle and calculates a speed and an acceleration of the vehicle by using an acceleration sensor equipped in the vehicle, and then adds the travel distance to a standard location to thereby calculate the present position and display the corresponding map and the position mark on the displayed map on the basis of the calculated present position. Incidentally, in the conventional process of calculating the travel distance, it is calculated by using a so-called vehicle speed pulse signal for a speed meter (i.e., a vehicle speed pulse signal having a predetermined pulse number for each tire rotation) to thereby count the pulse number.
On the other hand, the latter type receives measurement electric waves from a plurality of GPS satellites launched in the space, and then calculates the present position (i.e. the absolute position) of the vehicle by using a three-dimension survey method or a two-dimension survey method based on the received result, to thereby display the corresponding map and the position mark on the displayed map on the basis of the calculated absolute position.
The navigation apparatus using the latter GPS among them is characterized in that it is not necessary to input in advance a position of the self vehicle on the map and further a measurement error of the present position is extremely small, so that a high reliability can be obtained.
However, the navigation apparatus using the GPS type has such a defect that the measurement cannot be done when the self vehicle is located at the shade which causes the reception of the electric wave to be difficult, such as the shade of a tall building, the inside of a tunnel, the inside of a forest and the like. The self-sustained measurement type of the navigation apparatus has such a defect that it is easily subjected to the effects resulting from the factors inside and outside the vehicle, such as the effects of an accumulated error, a temperature variation and the like and thereby the output data is not always accurate.
Thus, a so-called hybrid type of a navigation apparatus is developed in which both of the GPS type and the self-sustained measurement type are used to mutually correct the respective defects.
It is known that in recent years, there are a plurality of roads extremely adjacent to each other associated with the development of a road network, in many cases. More actually, for example, there is a case that a general road runs parallel to and exists just under an elevated express way (i.e., a road dedicated to a car), or there is a case that at a grade separation, in addition to an overpass at the grade separation, a road as a side-road runs parallel to the overpass and exists under the grade separation.
Hence, it is necessary to properly consider and show the existing road even for a vehicle traveling through the location at which a plurality of roads having altitudinal differences are adjacent to each other.
However, the map displayed on the display is a basically flat map in the conventional navigation apparatus. Thus, the position of the vehicle is flatly considered in the navigation apparatus. In other words, it is displayed by considering only a positional relation when projecting the position of the vehicle onto a horizontal surface.
Hence, when the plurality of roads having the attitudinal differences are extremely adjacent to each other (e.g., the roads are overlapped on a flat surface, depending on a condition), it is difficult to exactly determine a road on which the vehicle is traveling, by the measurement accuracy of the current navigation apparatus. This results in a problem that an erroneous guidance may be performed.
This problem is a serious problem which has a great influence on an accuracy of a navigation process, in the present day when the roads are complicatedly developed as mentioned above.
By the way, the conventional hybrid type of the navigation apparatus uses an output of the moving distance sensor as it is, as the moving distance of the vehicle. However, when considering the actual movement of the vehicle, there are many cases of the travel on an upward slope and a downward slope as well as the movement within a horizontal surface. At this time, a map displayed on the display picture is normally flat (two-dimension) with a horizontal surface as a standard. Thus, in a case of the upward slope or the downward slope, the actually moving distance becomes longer than a moving distance on the two-dimension flat plane (within a horizontal surface). Hence, if in this case, as mentioned above, the output of the moving distance sensor as it is, is used as the moving distance of the vehicle and then the position mark is displayed on the display picture, the display position of the position mark on the map is far away from the actual present position. This results in a problem that the position display and the guidance cannot be performed exactly.
Therefore, the conventional vehicle navigation apparatus is equipped with an inclination instrument for detecting an inclination of the vehicle, and uses the detected inclination and then corrects the moving distance and the like and thereby guides the vehicle.
As an example of the inclination instrument, two piezo type semiconductor acceleration sensors may be prepared such that a sensitive axis of one acceleration sensor is coincident with a travel direction of a vehicle and a sensitive axis of the other acceleration sensor is mounted in a direction vertical to the travel direction of the vehicle, for example, as disclosed in Japanese Patent Application Laying Open (KOKAI) No.8-297033 (especially, FIG. 3 and the related explanation thereof). Then, the acceleration in the travel direction of the vehicle is determined by the acceleration sensor whose sensitive axis is coincident with the travel direction of the vehicle, and also an acceleration in a direction parallel to a gravitational acceleration is calculated by the other acceleration sensor whose sensitive axis is vertical to the travel direction of the vehicle, and further the calculated acceleration in the direction parallel to the gravitational acceleration is compared with the gravitational acceleration, and accordingly the inclination of the vehicle is determined.
However, the technique disclosed in the above mentioned Japan Patent Application Laying Open (KOKAI) No.8-297033 requires the two (two-axis) acceleration sensors. Thus, this results in another problem that the configuration of the whole navigation apparatus is complex and the price is expensive.
Moreover, a vibration and the like associated with the movement of the vehicle are superimposed on the output signal of the acceleration sensor as a noise component, in the acceleration sensor for detecting the gravitational acceleration. Hence, this results in another problem that the process with a complexity and a high level is required in order to remove then noise component.
Furthermore, the mount of the two acceleration sensors requires that the sensitive axes thereof are orthogonal to each other at an extremely high accuracy. The orthogonal accuracy is poor in the actual vehicle. Hence, this results in another problem that the exact inclination angle may not be calculated.